The gas-treatment or drying of light particles, by which is meant particles with a high surface density or ratio of surface area to mass, with a stream of a drying gas is difficult because an active and effective gas stream will entrain the particles, but a slow gas stream will not work fast enough.
Accordingly, it is known, as for example from U.S. Pat. No. 3,500,552 of D. F. Farkas et al, to dry wet particulate food by feeding the particles into a foraminous drum that is centered on and spun at high speed about a horizontal axis. This creates a centrifuge effect which effectively makes the particles heavier. Thus a high-speed stream of drying gas can be passed through the particles to dry them rapidly without entraining them.
In such a system it is normally necessary to use considerable centrifugal acceleration, normally from 50 meters/second.sup.2 to 500 meters/second.sup.2, so that a drying-gas stream can flow at an effective rate through the body to dry the particles and partially fluidize it. In machines with horizontal axes, however, this acceleration is increased by the acceleration of gravity, some 9.81 meters/second.sup.2, for the particles on the bottom and decreased by it at the top, so that a halting and pulsating movement of the body in the drum is produced.
It is also known, as for example from earlier commonly assigned U.S. Pat. No. 4,130,944 of G. Hultsch et al, to rotate the foraminous drum about an upright axis. The particles are introduced at the bottom of this drum and the gas stream is also moved from bottom to top. Thus the body will move up along the inner surface of the upright drum.
The problem with this style of operation is that when a batch is started there is no bed of particles covering the upper portions of the drum, so that resistance to flow through these upper portions is much lower than the resistance to flow at the lower portion which is covered internally by particles. At low rotation speeds it is difficult to form a body covering the entire inner surface of the drum, but at high speeds a much greater flow of drying gas is needed to fluidize the bed, and such fow is difficult to maintain when a major portion of the drum is offering very little resistance to flow. The normal procedure thus simply keeps feeding in the particles until the drum is virtually full, and then cutting down input until it matches output for steady-state operation.